1. Field of Invention
The present invention relates to an antenna structure. More particularly, the present invention relates to a helix antenna on a dielectric core.
2. Description of Related Art
Antenna is a necessary element in wireless communication. Usually, the radio-frequency (RF) signals, carrying information, are transmitted or received at the antenna. Then, the RF signals are further processed to obtain the actual information be carried. Due to the design of the signal processing circuit, the RF signal in EM-wave has its desired polarization state. Usually, it is a linear polarization or a circular polarization. The polarization state can be generated by a specific antenna structure.
In the modern life, mobile phone has been a very popular wireless communication apparatus. The mobile phone also needs to use the GPS to identify the position. In order to reduce the volume of the GPS unit, the size of antenna is necessary to be reduced. Particularly, if the volume of the mobile phone is intended to be reduced, the antenna size is also necessary to be reduced.
The basic antenna structure for generating the circular polarization RF signals has been well known in the art. FIG. 1 is a basic conventional structure of antenna in circular polarization. In FIG. 1, two antenna line structures 100, 102 are composed together in perpendicular crossing. The two antenna line structures 100, 102 are implemented on a straight signal cable 104. The two antenna line structures 100, 102 are respectively and electrically coupled to the signal cable 104 at the top end 106. The two antenna line structures 100, 102 at the other end 108 are commonly connected. By adjusting the length of the antenna in difference for the two antenna line structures 100, 102, a circular polarization can be obtained.
The antenna structure in FIG. 1 is rather conventional. The operation is not further described in detail. Since the antenna structure in FIG. 1 used no dielectric material, the size is large to produce the desired wavelength. FIG. 2 is another conventional design of helix antenna. This conventional antenna product is a type of Quadrifilar Helix Antenna (QHA). Basically, two pairs of antenna lines 120 are in helix structure and are formed on a dielectric rod core 122. The input impedance Zin of the antenna is about 1–5 ohms when using a dielectric core with high dielectric constant. However, the usual signal cable impedance is 50 ohms in the signal processing side. A long low-impedance cable 124 is thus needed for impedance transformation. This kind of cable is rather expensive. Also and, this antenna needs a large balun transformer 126 at the bottom side opposite to the signal coupling end. In this manner, since the balun transformer 126 occupies some space, the size of antenna cannot be further reduced.
In order to have better performance of antenna but with rather reduced size, some other designs of antenna are still under developing. Manufacturers still strongly intend to design an antenna with sufficient function but reduced size. Then, the antennal can be widely used in various wireless communication apparatus.